The present invention relates to a dipole transmission system and method for use in gas and oil wells. More particularly, the present invention relates to a dipole transmission system having one or more uphole assemblies and a single downhole assembly connected by a wireline and short hop data link enabling data transmission from the downhole assembly to the uphole assembly.
In the process of drilling an oil well, quite often it is desirable to drill the first section of the well vertically from the surface. When the bore hole is positioned near the oil producing formation strata, a deviated bore hole may be drilled in a non-vertical or horizontal direction. Deviation of the borehole is desirable so as to expose more of the bore hole to the oil producing formation.
In other cases it is desirable to re-complete existing producing oil wells by drilling new sidetracks extending out horizontally or at an angle from the existing vertical bore hole. Producing wells are typically cased with a steel lining. To enable a sidetrack to be drilled, a window is first cut in the casing to allow the drill bit and drill string to advance from the cased vertical hole into the formation.
In either of the above cases the direction of the borehole deviation or sidetrack must be measured and transmitted to the surface as drilling proceeds. It is also often desirable to measure and transmit to the surface other data concerning the borehole physical conditions such as temperature, pressure, etc.
A known method of transmitting downhole data to the surface is the use of an electric dipole transmitter, which functions by applying a phase modulated low frequency voltage across an electrically insulated section of the drill string (a gap sub). The applied voltage causes electric currents to be injected into the downhole formation. The transmitting gap sub is normally mounted downhole 10 to 20 meters behind the drill bit. The electric dipole method of transmitting data to the surface has many advantages over alternative methods (e.g. mud pulse telemetry), namely, higher speed, higher reliability due to the absence of moving parts, and lower operating cost.
If the formation resistivity from downhole to the surface is in a moderate range (typically 0.5 to 20 ohm-meters) the downhole injected currents can usually propagate to the surface where they can be detected by electrodes driven into the ground and connected to the top of the drill string. Such is not the case when the working liquid (mud) has a high content of gas. Overly gaseous liquids reduce the intensity of the returning signal to an undetectable point. Also, if the formation resistivity near the gap sub or in formation strata above the gap sub is very high or very low, the injected formation currents may not propagate to the surface with enough strength to provide a detectable signal.
An additional factor affecting the dipole signal strength at the surface is the depth of the transmitting gap sub. As the borehole depth increases, the dipole signal strength at the surface decreases and at some point becomes too weak to reliably detect.
The ability to work with non-Newtonian liquids (liquids in which the viscosity changes with the applied shear stress) containing high levels of gas is an obvious application for working with underbalanced systems. It is a desirable goal to develop methods of overcoming the depth and formation resistivity limitations of the electric dipole transmission methods discussed above.